1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. The semiconductor device in this specification means with which a thin film transistor is provided.
2. Description of the Related Art
In recent years, technological development of an SOI (silicon on insulator) substrate in which a single crystal semiconductor layer is formed over a substrate having an insulating surface has been promoted. Integrated circuits formed using an SOI substrate, which generally enable the parasitic capacitance of a transistor to be smaller than that in the case of using a silicon substrate, have been attracting attention in that they contribute to improvement of the operation speed and reduction of the power consumption. Therefore, applicability to high-performance semiconductor devices such as microprocessors and electronic devices such as ICs (integrated circuits) and electro-optical devices has been considered.
As an SOI substrate, for example, a semiconductor device in which a single crystal silicon layer is provided over an insulating substrate of heat-resistant glass or the like is known (see Patent Document 1). This semiconductor device has a structure in which a silicon layer obtained by a hydrogen ion implantation separation method is attached onto an insulating silicon film which covers the entire surface of a crystallized glass substrate having a distortion point of 750° C. or more.
It is demanded that electronic devices such as electro-optical devices have high reliability and that high productivity and less production cost are provided for a production method thereof. As a method for increasing productivity and reducing production cost, simplification of the manufacturing process is given.
In electronic devices such as electro-optical devices, thin film transistors are mainly used. In manufacturing thin film transistors, reduction in the number of photomasks used in photolithography is demanded for simplification of the manufacturing process. As the number of photomasks increases by one, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like, and moreover other steps before and after the aforementioned steps, such as film deposition and etching and further resist removal, cleaning, drying, and the like. Thus the increase in the number of photomasks in the manufacturing process by one results in the significant increase in the number of steps. Therefore, many techniques for reducing the number of photomasks in a manufacturing process have been developed (for example, see Patent Document 2).
Many conventional techniques for reducing the number of photomasks use a complicated step such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. There has been a concern that yield is reduced due to various problems caused by such a complicated technique. Moreover, electrical characteristics of thin film transistors have been sacrificed in many cases.
As typical means for reducing the number of photomasks in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. A technique for reducing the number of manufacturing steps by using a multi-tone mask is given in, for example, Patent Document 3.
[Reference]
    [Patent Document 1] Japanese Published Patent Application No. 2000-124092    [Patent Document 2] Japanese Published Patent Application No. 2003-179069    [Patent Document 3] Japanese Published Patent Application No. 2007-227440